The present invention relates to solids extraction from a liquid/solid slurry, and more particularly, to a sieve separator and a method of slurry separation that uses two or more levels of filtration screens. In some embodiments, different spacing in the filtration screens on different levels may be used to separate solids of various sizes at each level.
Manure produced at a feedlot or dairy barn must be removed regularly to ensure adequate sanitation and to prevent disease. One manure removal method is to flood the feedlot or barn floor with water, in order to wash away the manure. The resulting effluent is a liquid/solid manure slurry that typically is directed through pipes or channels to a manmade slurry pond, where the solids and particulates settle to the bottom of the pond. The liquid may be drained and recycled, and the solids may be removed and used as fertilizer.
There are at least two significant types of problems associated with the conventional use of slurry ponds as described above. One set of problems is generally associated with the high proportion of solids in the unseparated slurry, which may clog or otherwise impede the slurry as it passes through the channels or pipes to the slurry pond. Also, assuming that the slurry reaches the slurry pond, it is time-consuming and expensive to remove the significant proportion of solids that have subsequently settled to the bottom. The monitoring and removal of these solids from the bottom of the ponds forces workers to work for extended periods of time in an environment where large amounts of methane, ammonia and sulfur are produced by the decomposing manure. These gases not only create potentially dangerous working conditions, but also contribute to the pollution of the environment.
Another set of problems associated with the conventional use of slurry ponds is related to the prolonged mixing of solid and liquid wastes in the pond. This tends to result in the transfer of a significant amount of nutrients from the solid to the liquid, so that the solids become depleted of nutrients that may be desirable in a manure fertilizer, and the liquids become loaded with nitrogen, phosphorous and salts to such an extent that they must be either limited in their irrigation use, or mixed with fresh water to lower the proportion of these substances. In other words, the useful qualities of both the solid and liquid portions of the slurry mixture are degraded over time in a conventional slurry pond.
To solve these problems, machines have been developed that attempt to remove a significant proportion of the solids in manure slurry, prior to placing the slurry in a pond. One such machine, which will be referred to as a sloped-screen slurry separator, uses a sloped, wedge wire filtration screen to filter out the solids. In such a separator, the filtration screen has apertures configured to stop the passage of solids that exceed the aperture size, and to allow the passage of solids smaller than the aperture size. In this context, “sloped-screen” refers to the fact that the filtration screen is configured to have a non-horizontal orientation with respect to the surface of the ground, so that gravity may assist in the flow of the manure slurry over the filtration screen. A single filtration screen may include apertures of various sizes, to filter out different sized solids at different positions along the screen. In general, however, a filtration screen as referred to herein is any device configured to filter out some portion of solids from a mixture of solids and liquids.
An example of the type of slurry separator known in the prior art is found in U.S. Patent Publication Number US 2001/0013492 A1, entitled SLOPED SCREEN SEPARATOR THAT REMOVES SOLIDS FROM A MANURE SLURRY, filed on Mar. 30, 2001, the disclosure of which is incorporated herein by reference. Other examples of similar types of separators are found in U.S. Pat. Nos. 3,875,319; 3,919,433; 3,966,607; 4,018,899; 4,117,175; 4,666,602; 5,268,100; and 5,720,393, the disclosures of which are incorporated herein by reference.
However, such sloped-screen separators may not remove a sufficient proportion of the solids from the manure slurry in a single separation attempt. Therefore, multiple separators must sometimes be used, with the manure slurry passing from one to another in series. Since each separator requires its own construction, pumping and mounting provisions, the use of multiple separators in this manner entails a greater cost than the use of a single separator. Furthermore, transporting the partially separated slurry from one separator to another may lead to problems of pipe clogging as described previously.
In previous sloped-screen separators, a concentrated flow of slurry is typically pumped to a fine filtration screen, leading to almost immediate partial clogging of the filtration screen. The result is that pressure quickly builds up near the bottom of the sloped filtration screen, causing the slurry to back up and possibly overflow the sides of the filtration screen and/or the separated manure solids on the filtration screen. In particular, “streamers” occur when a section of filtration screen gets partially clogged, causing the slurry to run over the top of the filtration screen instead of filtering through it. This in turn causes the slurry to flow directly into the separated solids stack further down the sloped filtration screen, undermining the purpose of the separator.
Furthermore, it is often difficult to clean the filtration screens of previous sloped-screen separators. One reason for this is that in existing separators, fine solid particles typically reside at the bottom of each solids stack. Such fine particles are more fluid that larger solids, so that the resulting stack may be relatively unstable and difficult to remove efficiently. Existing separators are therefore often operated while partially clogged with solids, leading to a degradation of their functionality, and increased downtime and expense for frequent cleaning attempts.
Due to the problems outlined above, there is a need for a manure slurry separator that efficiently separates solids from liquids without the undue expense of constructing multiple sloped-screen separators, that avoids frequent clogging, and that can be relatively easily cleaned.